To date, for the detection of gamma and X-radiation, particularly in CT and dual energy CT systems, inter alia direct-conversion detectors based on semiconducting materials, for example CdTe, CdZnTe, CdTeSe and CdZnTeSe, have been used. In order to generate the necessary electrical properties for detector operation, these materials are doped. In general one of the elements Cl, In and Al are used for this, or a combination of at least one of the elements of the third main group with at least one element of the eighth subgroup of the periodic table.
Owing to the intrinsic defects specific to the growth process of the semiconductor materials, and their effects on the charge transport, it has not however yet been possible to convert high radiation densities, such as are employed for example in computed tomography, directly and without loss into electrical pulses. The reason for this is that polarization occurs. Polarization is intended to mean the reduction of the electric field by fixed charges bound to generally deep defects, which can then capture the charge carriers generated by radiation and recombine with them. They can therefore no longer contribute to the electrical pulse which is induced on the electrode on lower side of the detector, so that a significantly lower intensity of the radiation is suggested.
The effective mobility of the charge carriers is significantly reduced owing to the captured charge carriers. A radiation detector, however, must have a high charge carrier mobility so that the electrons and holes formed during the irradiation can be separated, so as to avoid the formation of a space charge in the detector and the polarization effect caused by this. The polarization accordingly limits the maximum flux detectable by a direct-conversion detector.
At present, no way of preventing the polarization effect in a direct-conversion detector is known in the prior art. This thus means that measurement of high radiation fluxes, such as are encountered for example in a CT system, is currently not possible by way of direct-conversion detectors.